Loss-of-function mutations in two genes, KRIT1 and OSM, cause Cerebral Cavernous Malformations (CCM). Despite the identification of the genetic basis for familial CCM, the function of these genes in vascular biology and disease are largely unexplored. OSM is most extensively studied and encodes an intracellular protein that is essential for mammalian cells to cope with physiologic stress. Our preliminary studies indicate that OSM and KRIT1 are critical scaffold proteins that regulate the response of endothelial cell-cell interactions to permeability factors and other stresses via a JNK kinase pathway. Specific Aim 1) Determine whether KRIT1 and OSM genes mediate developmental angiogenesis through an endothelial autonomous mechanism. These studies seek to establish that OSM and KRIT1 play critical roles in a common intracellular endothelial signaling pathway required for angiogenesis. Specific Aim 2) Investigate whether OSM and KRIT1 are endothelial scaffold proteins that regulate the integrity of endothelial cell-cell junction via a JNK-dependent pathway. We will examine whether OSM and KRIT1 forms an intracellular scaffold for an endothelial signaling complex that regulates JNK and curbs endothelial leak induced by vascular permeability factors and inflammatory cytokines. Specific Aim 3) Understand the mechanism of endothelial barrier dysfunction in OSM and KRIT1 deficient mice. We will test a model of CCM pathogenesis whereby a primary defect in endothelial tight junctions predisposes the cerebral vascular bed to inflammation and leak. At the conclusion of these studies, we hope our contribution will be the elucidation of a new intracellular signaling pathway regulating endothelial cell-cell interaction and provide one of the first examples of a hereditary cerebral vascular disease being caused by a primary defect in pathways that regulate endothelial barrier function. PUBLIC HEALTH RELEVANCE At the conclusion of our studies, we hope our contribution will be the elucidation of new intracellular signaling mechanisms regulating endothelial cell-cell interaction and the demonstration that hereditary cerebral vascular disease can be caused by a primary defect in molecular pathways that regulate endothelial barrier function.